Projection, printing, and scanning systems often require the formation of a light distribution that is uniform along a line or over an area. As is well known to those skilled in the optical arts, such a uniform light distribution may be formed via an optical system that comprises lenses that are acylindric (a term used for cylindrical lenses that are aspheric) or other aspherical glass or plastic optical elements.
Traditional grinding and polishing or glass molding processes are well suited for forming conventional lens shapes, such as spherical optical surfaces, radially symmetric aspheres, and flat surfaces. These lens fabrication processes typically yield surface roughness on the order of about 0.7 nm RMS. Traditional grinding and polishing procedures are also used for mold preparation, yielding glass-molded or plastic-molded parts that exhibit surface roughness in the same overall range. However, less conventional shapes, such as acylindric shapes, are not as easily fabricated using these traditional grinding and polishing or molding processes.
Eliminating or minimizing surface defects of optical components is considered to be of critical importance for many types of imaging and laser applications. Lens surface features generated by cutter marks are classified as mid-spatial frequency errors due to their relative size and pitch, and are acknowledged as a significant problem source for UV, visible, and IR applications. Propagation of wavefront errors due to mid-spatial frequency effects can cause unacceptable intensity modulations, even creating potentially damaging hot spots in the beam path for some types of optical systems. Errors at these frequencies can degrade beam quality beyond acceptability in many types of applications and may even lead to catastrophic system failure in extreme cases.
Chief among the problems caused by surface roughness of a lens element or a mirror is unwanted diffraction of light from an unpolished surface. Diffracted orders of light scattered by roughness at the surface interfere with each other as they propagate, forming undesired structures in the light intensity. Because of such effects, an unpolished lens or mirror having periodic surface roughness may be unacceptable for conventional optical applications.
As is well known to those skilled in optical fabrication, polishing and finishing techniques for acylindric lenses are considerably more challenging than the techniques required for finishing rotationally symmetric surfaces. Providing precision molded acylindric surfaces with a 0.7 nm RMS roughness typically requires one or more iterative processes. For example, a precision acylindric mold can be fabricated for initial molding of acylindric structures. The acylindric mold is then polished to form the molded element as a finishing step; this final polishing step is generally performed by hand, by a skilled master optician. As is well known to those skilled in optical fabrication, polishing procedures used to achieve the required surface characteristics must be executed with extreme care, lest the original acylindric shape itself be lost. Moreover, any tooling used to figure the acylindric shape may leave process-induced roughness in the optical component. Similar difficulties arise whether the lens is ground and polished in glass, or molded, or fabricated using a combination of molding, figuring, and finishing techniques. Even where satisfactory surface smoothness is achieved, these fabrication complications can cause an optical component to be prohibitively expensive, especially for apparatus in a prototype stage.
Conventional lens polishing and finishing techniques, used for spherical and plano surfaces, have been successfully adapted for some types of basic non-axisymmetric shapes such as prisms and cylindrical shapes. However, complex acylindric shapes have proved more difficult to polish, particularly for smaller optical components. Thus, it can be appreciated that there is a need for optical design techniques that enable the effective use of acylindric components and other irregular lens structures, without restrictions imposed by the inherent limitations of conventional lens polishing. There is also a need for methods that allow lower cost fabrication of aspherical and other optical elements, particularly for prototyping and low-volume applications.